This invention is directed to insulation for power cables. More particularly, this invention is directed to insulation for high voltage direct current power cables.
Direct Current (DC) power transmission has several advantages over alternating current (AC) power transmission. DC current transmission does not have a length limit, permits long-distance submarine cables ( greater than 50 km), has good connectivity among different networks/sources (such as, windmills), has lower operating costs due to low conductor loss and no power loss, has superior power quality and flow control for system reliability/stability and has higher voltage ratings. Cables insulated with oil/paper insulation have been successfully used for high voltage direct current (HVDC) applications since 1954. Cables insulated with crosslinked polyethylene can have several advantages over cables insulated with oil/paper for HVDC applications. The advantages of crosslinked polyethylene include lower manufacturing costs, lower operation costs, easier maintenance for utilities, higher temperature ratings (such as, 90xc2x0 C. vs. 60xc2x0 to 70xc2x0 C.) to utilities, and environmental friendliness due to no oil leakage.
Polymeric dielectric insulating materials, particularly polyethylene without modification, however, can not be used for HVDC applications. These materials have local space charge buildup which can significantly enhance local fields under surge or lightning impulse, have charge neutralizations during reverse polarity which can reduce local DC breakdown strength, and have stress inversions due to temperature-dependent conductivity which can reverse local field enhancement.
A known approach to develop HVDC polymeric cable insulation products has been to have low and well-distributed space charge traps. Space charge can be trapped by physical traps formed between crystallinity and amorphous boundaries or chemical traps due to chemical structures of substances. The instant invention, however, is a cable insulation made from a blend which includes an ethylene copolymer, such as an ethylene-alpha olefin copolymer with low crystallinity to reduce physical space charge trapping sites. The invention uses at least one polar polymer modifier in an effective amount to enhance local conductivity to leak space charge quickly when local stress is enhanced, and at least one ion scavenger to stabilize or neutralize the space charge to provide a composition which is an effective high voltage DC cable insulation.
The invention is directed to (1) a direct current cable which includes insulation which resists breakdown and deterioration when exposed to high-voltage direct current, (2) an insulation composition which resists deterioration and breakdown when exposed to high-voltage direct current, and (3) a method for reducing the deterioration of such insulation. The cable insulation composition includes at least one crosslinked non-polar, low crystallinity resin with a density of less than 0.900 g/cc which tends not to trap charge or create charge trap sites for a cable insulation temperature rating of at least 90xc2x0 C. In another aspect, the resin is not crosslinked or is crosslinked only in a low amount (hereinafter a non-crosslinked polymer) which is effective for providing a cable insulation with a temperature rating of 75xc2x0 C. or above. In either aspect, the cable insulation also includes (1) at least one polar polymeric modifier which dissipates or leaks charge quickly under high fields, (2) at least one ion scavenger which stabilizes or neutralizes space charges, and (3) optionally at least one heat stabilizer which minimizes internal charge generation during in service thermal degradation of insulation.
The crosslinked non polar low crystalline resin, polar polymeric modifier, ion scavenger and heat stabilizer are in amounts effective for achieving temperature rating of 90xc2x0 C. or above, a charge density less of than 2 Coulomb/mm3 measured by a pulsed electro acoustic (PEA) method after 24 hours with either positive or negative 20 kV/mm applied. For the cable insulation which has a temperature rating of not more than 75xc2x0 C. the amount and extent of crosslinking of such resin, the amounts of polar polymeric modifier, ion scavenger and heat stabilizer all are effective for achieving temperature rating of 75xc2x0 C. or above, a charge density less of than 2 Coulomb/mm3 measured by a pulsed electro acoustic (PEA) method after 24 hours with either positive or negative 20 kV/mm applied.
In another aspect, the invention is a high-voltage direct current cable insulation composition which has a temperature rating of 90xc2x0 C. or above and which comprises a blend of or which is made from a blend of at least one cross-linked ethylene copolymer, such as ethylene/alpha olefin polymer, having a density of less than 0.900 g/cc, a melt index of from 0.5 to 10 g/10 minutes, a crystallinity of less than about 10%; at least one polar polymeric modifier in an amount effective to provide field conductivity and permitting leakage of space and charge only at high fields; at least one ion scavenger in an amount effective to reduce charge build-up relative to a blend which does not include an ion scavenger; and, optionally, at least one heat stabilizer in an amount effective to prevent thermally induced degradation and resulting internal charge generation. The polar polymeric modifier, ion scavenger, and optional heat stabilizer are in amounts and ratios which when in combination with the crosslinked resin provide the insulation with a charge density less than 2 Coulomb/mm3 measured by a PEA method after 24 hours with either positive or negative 20 kV/mm applied.
In another aspect, for cable insulation which has a temperature rating of 75xc2x0 C. or above, the cable insulation composition comprises a non polar, non-crosslinked ethylene copolymer, such as an ethylene/alpha olefin copolymer, having a density of less than 0.900 g/cc a melt index of from 0.5 to 10 g/10 minutes, a crystallinity of less than about 10%; at least one polar polymeric modifier in an amount effective to provide field conductivity and permitting leakage of space and charge only at high fields; at least one ion scavenger in an amount effective to reduce charge build-up relative to a blend which does not include an ion scavenger; and, optionally, at least one heat stabilizer in an amount effective to prevent thermally induced degradation and resulting internal charge generation. The polar polymeric modifier, ion scavenger, and optional heat stabilizer are in amounts and ratios which when in combination with the resin provide the insulation with a charge density less than 2 Coulomb/mm3 measured by a PEA method after 24 hours with either positive or negative 20 kV/mm applied.
In yet another aspect, the invention is a high-voltage direct current cable insulation which comprises a blend of or which is made from a blend of at least one crosslinked ethylene-butene or hexene olefin polymer having a density of less than 0.900 g/cc, a melt index of from 0.5 to 10 g/10 minutes; from 0.1 to 15 weight percent of at least one polar polymeric modifier; from 0.05 to 0.5 weight percent of at least one charge scavenger to reduce charge build-up, and optionally, from 0.1 to 5 weight percent of at least one heat stabilizer in an amount effective to prevent thermally induced degradation and resulting internal charge generation.